1. Field of the Invention
The invention relates generally to the field of photography, and in particular to film cassettes. More specifically, the invention relates to a film cassette having an internal sprocket which when rotated in engagement with a film leader propels the leader from the film cassette.
2. Description of the Prior Art
Prior art Jap. Kokai No. 3-80243, filed Aug. 24, 1989 as Application No. 1-217860, and published Apr. 5, 1991, discloses a film cassette comprising a lighttight housing, a spool rotatably supported inside the housing, a filmstrip wound onto the spool, a film leader attached to a leading end of the filmstrip, and a sprocket coaxially connected to the spool with identical pitch teeth adapted to be received in respective perforations along an edge section of the leader wider than the filmstrip for advancing the leader (but not the filmstrip) from the housing when the sprocket and the spool are rotated in a film unwinding direction. The perforations in the leader each have the same length. Consequently, when the leader is wound onto the spool after the filmstrip by action of the filmstrip pulling in the leader, it appears difficult for anyone of the teeth to align with the perforation closest to the filmstrip. Thus, the teeth and the perforations may not engage one another. Moreover, since there will result several convolutions of the leader, the increased diameter of an outer convolution as compared to an inner convolution makes it difficult to place the teeth in the perforations along the outer convolution after they are received in the perforations along the inner convolution.
Prior art U.S. Pat. No. 1,921,560, issued Aug. 8, 1933 discloses a film spool having a single radially projecting pin that extends through aligned holes in respective convolutions of a filmstrip coiled about the spool core. The spaces between the holes differ to allow the holes to be aligned to receive the pin as the filmstrip is wound onto the spool core.